U.S. Pat. No. 4,813,989 describes a method for a closed-loop control of a diameter. A defined amount of gas is here blown into a pressure control chamber via a blower in order to influence the pressure conditions in the tube so that the outer diameter of the tube remains constant during the internal deposition.
JP 62012181 describes a method where pressure regulation is used in only one direction. The pressure is correspondingly maintained constant in the return flow. Regulating the pressure in the forward flow also aims at maintaining a constant outer diameter.
JP 3052418 describes a method for maintaining a constant internal pressure within the substrate tube during deposition. In the process, a valve is used on the exhaust side of the installation for regulating the pressure.
A combination of the aforementioned methods is described in EP 1719739. The closed-loop pressure control is here implemented through several partial currents. These partial currents can be provided both on the gas inlet side and on the outlet side. At least one partial current is merely open-loop-controlled and at least one partial current is closed-loop-controlled. An increase of the internal pressure of the pipe from the beginning to the end is achieved along the length of the pipe, and the closed-loop control allows maintaining a constant outer diameter along the length of the pipe, so that the parameters of the fiber obtained from the blank mold show few deviations. An introduction of glass soot from the discharge system can also be minimized.
US 2013/081430 describes a method where the substrate tube is inflated during at least a part of the internal deposition. Advantageous effects on the geometry of the substrate tube are achieved by increasing the outer diameter so that a twisting and deflection of the substrate tube can be reduced. This results in a more uniform deposition so that the fiber parameters stemming from the warped blank mold can be maintained within a significantly narrower tolerance range.
Most of the mentioned methods are adapted to maintain a constant diameter of the blank mold during the coating process. However, the outer diameter has almost no influence on the deposition that occurs inside the tube. These methods therefore lead to undefined and/or strongly varying thicknesses and profiles of individual glass layers deposited on the inside. This results partly in considerable flaws in the refractive index profile. Multi-mode fibers manufactured in accordance with the prior art are therefore meteorologically characterized and correspondingly assigned to quality classes. In so doing, flaws in the refractive index profile have a considerable influence on the proportion of fibers of the individual quality classes. Fewer flaws in the refractive index profile and smaller refractive index deviations provides a higher yield of fibers in the desired quality classes.